Siloxane refers to compounds containing Si-O-Si bonds. Siloxane is divided into chain and cyclic, the general formula of chain is SinOn-1H2n+2, the structure is H(SiH2-O)xSiH3, if n=2, it is disiloxane, SiH3-O-SiH3, n =3 is propylsiloxane, SiH3-O-SiH2-O-SiH3, the cyclic general formula is (H2SiO)n, such as n=4 is cyclobutanesiloxane, (H2SiO)4. Their halogenated substances are called halogenated siloxanes. Alkyl substituents are organosiloxanes, called silicones.
Molecular formula: (R2SiO)x
Properties: A polymer with Si—O—Si bond to form the main chain structure. It is customary to call silicone or polysiloxane, and it can be a linear, cyclic or cross-linked polymer.
According to the product application, it is classified into three categories: silicone oil, silicone rubber and silicone resin.The raw material for the synthesis of siloxane is the derivative of silane SinH2n+2. Linear siloxane and cyclic siloxane can be obtained by hydrolysis of R2SiCl2 type organochlorosilane. Use R2SiCl2 and RSiCl3 type organochlorosilane to hydrolyze and polycondensate to obtain crosslinked siloxane.
Siloxane is stable to heat and chemical reagents, is not wetted by water, has outstanding aging resistance, excellent hydrophobicity and electrical insulation. Used as fire retardant, lubricant and moisture-proof agent. Silicone resin is used to make fiber-reinforced materials. Silicone rubber is also used in adhesives and sealing materials.
There are many types of polysiloxanes, which are mainly divided into three categories according to the chemical structure and properties of the products:
①Silicone oil (modified silicone oil) is a low molecular weight (generally less than 10,000) linear structure polymer. In order to achieve certain properties, the molecular structure will also be grafted and modified. Colorless or light yellow transparent liquid, water-repellent and oil-repellent, with high boiling point and low freezing point, good heat resistance and chemical stability, and is used in various industries as chemical additives; the most common one is that it can be used as Defoamers in various industries, hydraulic oils, advanced lubricants, filler surface treatment agents and demulsifiers, latex coagulants, and can also be used as waterproofing/water reducing agents for building materials, textiles, paper and many other materials, plastics Industry lubricants, release agents, personal care hair moisturizers, foam stabilizers in foamed polyurethane, agricultural ink coatings industry can also be used as leveling agents, gold-letters, etc., and also a good electrical insulator.
②Silicone rubber is a linear structure polymer with a large molecular weight (one hundred thousand grade). After adding fillers and other additives, and vulcanizing, an elastomer, namely silicone rubber, can be obtained. Silicone rubber has the characteristics of electrical insulation, chemical stability, physiological inertia, and good elasticity in a wide temperature range; it is mainly used to manufacture rubber products used at high or low temperatures and artificial organs required in Plastic surgery.
③Silicone resin is a linear structure polymer that contains active groups and can be further cured. It can make heat-resistant high-grade electrical insulating paint (silicone paint), heat-resistant plastic (silicone plastic), laminated glass cloth and protective coating, etc.
Polysiloxane is also called silicone. The synthesis of low-molecular-weight polysiloxane usually adopts a stepwise polymerization reaction. E.g. Dimethyldichlorosilane is hydrolyzed, dehydrated or dehydrochlorinated to form polydimethylsiloxane:
The product is a mixture of cyclized oligomer and linear polymer close to the same amount, and there is a balanced relationship between them. Depending on the reaction conditions, the content of cyclized compounds can reach 20% to 80%, mainly tetramers. Often after the initial equilibrium is reached, a capping agent [(CH3)3Si]2O is added to the reaction system to cap the linear polymer to stabilize the product ratio. The polymerization reaction can be carried out under acidic or alkaline conditions. The alkaline conditions are conducive to the formation of higher molecular weight polymers, and the content of cyclized products can be reduced by distillation under reduced pressure. The mixture product is liquid and can be used directly.
When the trifunctional monomer methyl trichlorosilane is added to the above reaction system, a non-linear polysiloxane can be formed. After the product is separated from the water layer, under the action of the catalyst zinc oxalate, the content of the cyclized compound can be reduced, and the molecular weight can be further increased. The silicone resin thus obtained is added with a basic catalyst and heated for cross-linking during application.
Silicone elastomers are divided into room temperature vulcanized silicone rubber (RTV) (degree of polymerization of 200 to 1500) and heat-cured silicone rubber (degree of polymerization of 2500 to 1 1000). The latter is obtained by ring-opening polymerization of cyclic monomers. Vulcanization and curing and cross-linking are synonymous, but the term curing has the meaning of further polymerization and cross-linking. One-component room temperature vulcanized silicone rubber is composed of hydroxyl-terminated polysiloxane, cross-linking agent (methyl triacetoxy silane) and catalyst (dibutyl tin laurate). The moisture in the air can make the cross-linking agent It is hydrolyzed into CH3Si(OH)3 and acetic acid. The former can be cured with polysiloxane at room temperature (Formula 2-155).Under the catalysis of chloroplatinic acid or platinum complex, Si-H and vinyl group undergo an addition reaction. Under the catalysis of tin salt, Si-H and Si～OH can also react, and H2 is released.Polysiloxane can maintain its physical properties in an abnormally wide temperature range (-100°C ~ 250C). Its T is as low as -127℃, so it has outstanding low temperature flexibility. Secondly, it is resistant to high temperature and oxidation, has good stability in chemical and physiological environments, and has good weather resistance, water repellency and dielectric properties. Polysiloxane has a wide range of uses. The liquid product is used as a defoamer, fiber hydrophobic finishing agent, surfactant, lubricant, vacuum grease and heating oil bath. Resins are used as varnishes, paints, mold release agents, adhesives and insulating materials. Elastomers are used as sealing materials, electrical insulating materials, gaskets and tubes, etc., and can also be used as medical materials, such as artificial heart valves, pacemakers, contact glasses, and plasma bottle inner coatings. Compared with other organic elastomer materials, the strength of silicone elastomer is relatively poor, so adding fillers to enhance it is more important.